1. Field of the Invention
The present invention relates to improved apparatus for developing latent electrostatic images with marking particles and more particularly to such apparatus of the donor roller type whereby single component developer is adhered to a cylindrical applicator and rotated in non-slip transfer relation with the image member to be developed.
2. Description of the Prior Art
Since the commercialization of electrophotography, numerous different general approaches have been devised for applying marking particles to a latent electrostatic image, i.e., developing the image. Each general approach has been implemented in a great many different devices, illustrating that development continues to present a problem in such copying devices. Cascade and magnetic brush development systems have met with the most commercial success; however, these systems involve two component developer and related problems such as concentration control and carrier particle carry-out on the image member.
Thus continuing attempts have been made to devise single component development systems. Perhaps the most frequently pursued general approach has been fur brush development, in which the bristles of animal fur or synthetic fibers, properly triboelectrically related to toner particles, charge and attract the toner in a reservoir and then apply the attracted toner to the image to be developed. Such brushes have been made partially electrically conductive, e.g., to provide a threshold below which toner will not transfer to background areas of the image. However, there are various problems with fur brush applicators, a substantial one being a difficulty in uniformly loading the brush and the non-uniformity of development which thus results.
Various attempts have therefore been made to develop other single component applicators. An approach pursued by a significant number of implementations has been a "donor roller technique" in which a layer of charged toner particles is adhered by some means to an applicator member that moves into a transfer relation with a moving electrostatic image member at a speed selected to eliminate any relative movement between the roller and the image member at the development zone. The problems which must be dealt with in such an approach are several. Layer thickness must be uniform to achieve uniform development, agglomerates of the toner powder should not be present in the layer and the toner particles should have substantially uniform charge. Additionally, it is very desirable to provide some threshold control to avoid the transfer of particles to background charge areas on the image member.
The present invention pertains to an improved implementation of this latter approach which obviates such difficulties and provides excellent image development.